Fix artwork upload pipeline

backend/prisma/schema.prisma

@@ -17,6 +17,7 @@ model User {
   devices        Device[]
   tracks         Track[]
   audioAssets    AudioAsset[]
+  artworkAssets  ArtworkAsset[]
   uploadSessions UploadSession[]
   libraryEvents  LibraryEvent[]
 
@@ -46,7 +47,7 @@ model Track {
   id                  String        @id @default(uuid()) @db.Uuid
   userId              String        @db.Uuid @map("user_id")
   primaryAudioAssetId String?       @unique @db.Uuid @map("primary_audio_asset_id")
-  artworkAssetId      String?       @unique @db.Uuid @map("artwork_asset_id")
+  artworkAssetId      String?       @db.Uuid @map("artwork_asset_id")
   title               String
   artist              String
   album               String?
@@ -96,16 +97,20 @@ model AudioAsset {
 }
 
 model ArtworkAsset {
+  userId        String?  @db.Uuid @map("user_id")
   id            String   @id @default(uuid()) @db.Uuid
-  sha256        String   @unique
+  sha256        String
   storageKey    String   @unique @map("storage_key")
   mimeType      String   @map("mime_type")
   width         Int?
   height        Int?
   fileSizeBytes BigInt   @map("file_size_bytes")
   createdAt     DateTime @default(now()) @map("created_at")
-  track         Track?   @relation("TrackArtwork")
+  tracks        Track[]  @relation("TrackArtwork")
+  user          User?    @relation(fields: [userId], references: [id], onDelete: Cascade, onUpdate: Cascade)
 
+  @@unique([userId, sha256])
+  @@index([userId])
   @@map("artwork_assets")
 }